In the treatment of comminuted cellulosic fibrous material suspensions--paper pulp--, particularly in the thickening, washing, and bleaching thereof, an effective commercial vessel is a pressure diffuser. A very successful pressure diffuser is that sold by Kamyr, Inc. of Glens Falls, N.Y. under the trademark "MC.RTM.." The invention can also be utilized with a double stage pressure diffuser such as in co-pending U.S. application, Ser. No. 07/718,294, filed Apr. 16, 1991; variable stage diffusers; a type of pressure diffuser such as shown in U.S. Pat. No. 4,944,167; or any other type of pressure diffuser.
A pressure diffuser is typically used for washing medium consistency pulp, e.g., having a consistency of about 6-15%. Pulp enters the bottom of an upright vessel and moves upwardly forming an annular mat between an interior screen and the vessel shell. At the top, the pulp is agitated by a scraper and is expelled from the vessel through a pulp outlet. As the annular pulp mat moves upwardly in the vessel, water from the wash water headers passes through the pulp mat to the screen, washing the pulp. A hydraulic system moves the screen upwardly at approximately the pulp upward velocity, and then downwardly quickly to clean the screen by wiping and backwashing.
While commercial pressure diffusers are very successful and worthwhile machines, there are some circumstances under which they can be clogged. Clogging of the pressure diffuser is a very serious situation, which requires that the vessel--and perhaps a number of components cooperating with the vessel--be shut down until the clog is cleared. Clogging typically occurs near the pulp inlet to the bottom of the vessel. Near the pulp inlet, a bottom support is provided for the screen, mounted centrally of the vessel, and defining with the interior wall of the vessel at that area an annular passageway. This annular passageway tapers in cross-section (getting progressively smaller) as the pulp moves from the pulp inlet up to an interior volume between the screen and the interior wall of the vessel.
In the past where clogging problems have been anticipated--such as when softwood pulp with long fibers is typically to be washed using the diffuser--the bottom support for the screen and the inner vessel wall surface defining the passageway have been polished to reduce friction. However, even this polishing does not minimize the chances that the diffuser will clog.
According to the present invention, the friction at a free annular passageway near the pulp inlet in a pressure diffuser is minimized by introducing lubricating fluid (typically water) into the annular passageway. The introduction of the lubricating fluid is preferably accomplished utilizing first and second pluralities of nipples disposed circumferentially around the annular passageway in the wall of the vessel defining the passageway. Each nipple is connected--preferably through a valve--to a source of liquid under pressure slightly higher than the pressure in the passageway, and is introduced at a flow rate such that the liquid flows upwardly along the wall of the vessel from its point of introduction between the pulp and the wall, thereby providing minimum friction of the pulp with the wall, and thus minimizing the possibility of clogging.
Typically the wall at the passageway comprises a curved portion, and a straight (vertical) portion above the curved portion. The first plurality of the nipples is disposed in the wall straight portion, and the second plurality of nipples is disposed in the wall curved portion. Preferably the nipples of the two sets are circumferentially staggered with respect to each other, and are each twelve in number so that a nipple is disposed approximately every 15.degree. around the circumference of the annular passageway.
Located just above, or at the level of, the screen bottom support, a baffle is provided associated with the wash system for facilitating the proper introduction of wash liquid. To further minimize clogging, according to the invention, that baffle is of stainless steel or a metal with a low friction interior surface coating. Also--as known per se in the prior art--the exterior surface of the screen bottom support and the vessel inner wall where it defines the passageway are both polished so that they are free of any significant surface manifestations and thereby provide low friction to the flow of pulp.
A common annular header which supplies lubricating liquid to the nipples preferably is mounted on generally horizontally extending plates connected to the support legs for the vessel, and held to the plates by U-bolts.
While the lubricating fluid introduced by the nipples is typically at a flow rate such that it only performs a lubricating function, and does not significantly dilute the pulp at the passageway, the nipples can be operated to perform a diluting function too. If an operator of the pressure diffuser senses slow down stroke of the screen (the screen is designed to move downwardly very quickly, and if it does not, screen cleaning is not as effective), the operator may believe that the slowed down stroke is a result of the pulp properties near the screen bottom support. In order to remedy this condition, the operator need only increase the flow rate of lubricating liquid (typically water) applied by the nipples so as to significantly dilute the suspension at the passageway until the slow downward stroke of the screen is overcome, at which time the original, only lubricating, flow rate operation of the nipples is restarted.
It is the primary object of the present invention to provide a pressure diffuser, and method of treating paper pulp in such a diffuser, that minimizes the friction between the pulp and the vessel adjacent the pulp inlet, to in turn minimize the tendency of the annular pulp passageway adjacent the inlet to clog. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.